Other stars planning to attend the event are actor Will Smith and wife Jada Pinkett Smith. Andrea Bocelli, the famous tenor from Tuscany, is expected to perform.

Bridesmaids and Flower Girls

But the ceremony itself may upstage the wedding's A-list attendees.

"People are fascinated by this Tom and Katie wedding, not just because they're celebrities but also because there is sort of this mysterious Scientology element to the wedding," said Katrina Szish, contributing editor of celebrity magazine Us Weekly. "People are used to any other type of religious wedding. But Scientology is always sort of shrouded in mystery."

According to Bob Adams, a vice president of the Church of Scientology, Scientology weddings aren't all that different from other types of nuptials.

"It is very traditional in its model. It's set up the way most traditional weddings are from various religions," Adams said.

A Scientology wedding looks much like a typical Christian one: bridesmaids on the left, groomsmen on the right.

The ring bearer and flower girls play a part. At the center of the wedding are the maid of honor, the bride, the groom, and the best man.

1950s Vows and Triangles Inside Rings

But the vows of the nuptials are in a category all their own. Scientology offers four types of ceremonies.

For Tom, the more traditional sermon would go like this: "Now Tom, girls need clothes and food and tender happiness and frills. A pan, a comb, perhaps a cat."

For Katie: "Hear well, sweet Katie. … For promise binds. Young men are free and may forget. Remind him then that you may have necessities and follies, too."

If the words sound like they're a throwback to the 1950s, that's because they are. That's when Scientology's founder, L. Ron Hubbard, wrote them.

ABC News has learned that Tom and Katie will likely choose one of the church's less traditional rituals, known as the double-ring ceremony.

In it, the minister holds up the couple's rings and asks that they imagine a triangle inside them.

"They're asked to imagine affinity, reality and communication as the points of those triangles," Adams said. "Affinity, reality, communication equal understanding. … To represent the long-lasting relationship that they will have."

Though there have been reports that the wedding -- whatever form it takes -- won't be legal because it'll be in Italy, sources tell ABC News that's simply not true.

Both actors took care of the legal paperwork at home in California before traveling to Italy.

After Saturday's wedding, they'll be married in the eyes of Scientology and the eyes of the state.